Search apparatus have been known for some time in various forms and constructions. Such apparatus serves in general for the detection and discovery of metal objects which are hidden underground. These operate generally on the principle that an alternating magnetic field from a search coil on embracing the metal objects induce eddy currents in the objects which currents build up a counterfield that acts upon the search coil. It is this reaction which produces signals in the receiver winding that are evaluated and provide an indication. The signals indicate corresponding to the physical characteristics of the object from which they are set in action, the characteristic differences existing especially in the phase relation. Therefore, phase relationship is often used for the classification of the kinds of discovered objects. Accordingly, the possession of a reference phase angle which is fixed with respect to the phase of the signals of sought objects can be of great importance.
Often, the useful signals are covered over by noise (disturbance) signals which can be greater than the signals of the sought after objects. In this instance, of particular importance is the soil which surrounds the sought after objects. On one hand, this soil can contain mineral components, which possess a magnetic permeability greater than 1. One refers to magnetizable soils which generate an alternating magnetic flow through the search coil and which produces a signal in the coil. On the other hand, salt water can, specifically on beaches, be the media which surround the sought after objects. In this connection, we are speaking of weak electrically conductive media acting on the search coil. The described soil effects, interestingly, result in noise signals which in a definite frequency range stand at 90 degrees to one another, and, in fact, the signals from magnetizable soils fall in the 90 degree direction and signals of electrically weak conductive media almost in the zero direction to the impedance plane. These stand in a fixed phase relationship to the signals of different search objects. This circumstance can be used in order to separate useful and noise signals from one another and to suppress the effect of noise signals on the search result. Crucial to the operability of such a disturbance signal suppression in after connected electronics is that there be provided a reference phase angle fixed in relation to the phase base of the noise signals and which is particularly not changed by phase drift in the energizing and receiving coils or in the evaluation electronics.